A full set of audio system comprises an audio source, an amplifier, and a loudspeaker. The audio source provides electrical signals to amplifier; then, the amplified electrical signals are delivered to the loudspeaker, which would then transform electrical energy to acoustic energy, i.e. the sound. Speakers are generally categorized into electrostatic loudspeaker (EFL), moving coil loudspeaker, and piezo loudspeaker. The most popular loudspeaker is the moving coil loudspeaker because of its simple structure, low cost, and better sound quality. A typical full set of moving coil loudspeakers is composed of a diaphragm, a permanent magnet, a moving coil, and a casing; the diaphragm is mainly responsible for creating sound in which sound quality significantly influenced by the diaphragm material. In other words, sound quality primarily depends on the accurate vibration of the diaphragm, and a ideal diaphragm can vibrate without distortion to generate sound through a wide range of vibration frequencies.
A satisfactory speaker should have at least the following three characteristics:                1. Speaker diaphragm should have a high Young's modulus (modulus of elasticity) to response faithfully by the moving coil.        2. The speaker diaphragm should have a high mechanical damping capacity (internal loss), so that the irregular mode of vibration can be regulated.        3. Speaker diaphragm material should be light weight for immediate responding with the moving coil.        
To accomplish the above-mentioned characteristics, various materials such as paper, metal, ceramics, and polymeric materials have been developed for use in making speaker diaphragm. A single speaker diaphragm material cannot fulfill the aforementioned characteristics to reach ultimate sound quality. Therefore, composite materials were considered for use. The typical example is using carbon fibers (with high Young's modulus) to strengthen the polypropylene resin for speaker diaphragm in practical industrial service. This allows the speaker diaphragm to have an increased elastic modulus and damping capacity at the same time; however, it is difficult to trade-off. Among the literatures available to the public, there are a variety of methods for improving sound quality from different aspects. One example in a Taiwanese patent (No. 201023660) disclosed speaker improvement concerning the decrease in the undesired vibration of the coil. The speaker comprises a frame and a vibration module connected to the frame and is composed of an edge and a diaphragm. The diaphragm is connected to the peripheral edge of the frame so that when the coil vibrates and starts making sounds, the edge helps stabilizing.
An U.S. Pat. No. 5,805,726A1 introduced a piezo loudspeaker that focuses on the improvement in vibration by using a damping force to acquire better sound quality; the speaker is compact in size, has high fidelity, and is free from interference of EM wave. With all the advantages, such a speaker should be considerably popular in the market. However, because of the difficult manufacturing process and the high manufacturing cost, manufacturers are having second thoughts in adopting this technology. Furthermore, because this speaker adopts a diaphragm driven with a single piezoelectric actuator, it causes a deficiency of sound pressure. As a result, the speaker diaphragm is inflexible, and the application is thus greatly limited.
Following the advancement of modern technology, innovations to materials engineering have led great evolution for speaker diaphragm. These improvements for speaker diaphragm can be dated back to 1998 in the U.S. Pat. No. 4,772,513, where demonstration was done to the improvement of sound quality by using an additional amorphous carbon material coated on the metallic diaphragm or the composite diaphragm to have a high Young's modulus and light weight. Another Taiwanese Pat. 201130329A1 disclosed a new metal titanium diaphragm with a diamond-like coating, which the sound quality is greatly improved; however, metal titanium diaphragm is only viable for operation in high sound frequency. On the other hand, carbon nanotube is a newly discovered nano material and was adapted as an additional coating supported by a polymeric membrane to form a speaker diaphragm in a Chinese Patent CN101288336A. However, the adhesion of the overall carbon nanotube layer can be poor, not to mention possible difficulty in homogeneous dispersion of the carbon nanotube.